AuraHandledException

From Apex class:

 @AuraEnabled

    public static List<Account> fetchAccount() {

        throw new AuraHandledException('User-defined error');

    }

From client-side controller:

doInit: function(component, event, helper) {

        var action = component.get('c.fetchAccount');

        action.setParams({ firstName : cmp.get("v.firstName") });

        action.setCallback(this, function(response) {

            var state = response.getState();

            if (state === "SUCCESS") {

                component.set('v.Accounts', response.getReturnValue());

            }

            else if (component.isValid() && state === "ERROR") {

                console.log("Error Message: ", response.getError()[0].message);

            }

        });

        $A.enqueueAction(action);

    }